The present invention relates to doping of semiconductor integrated circuits, and more particularly, but not exclusively, relates to a technique for maskless doping of an integrated circuit semiconductor substrate with a different form of dopant than an electrode structure projecting from the substrate.
The advance of integrated circuit (IC) technology toward faster, higher density integrated devices frequently focuses on decreasing device feature size. In the case of integrated Field Effect Transistors (FETs) such as Insulated Gate FETs (IGFETs), device shrinkage often favors shallow source and drain junctions. For p-type substrate source and drain regions, boron atoms are a common acceptor dopant. Unfortunately, the implantation of elemental boron (B11.sup.+) ions is generally unsuitable for the formation of shallow source and drain junctions.
In one attempt to solve this problem, boron difluoride (BF.sub.2.sup.+) ions are implanted into the substrate to deliver the boron atom dopant. The relative higher atomic mass of BF.sub.2.sup.+ ions results in a shallower junction penetration compared to B11.sup.+ ions implanted at the same implantation energy level. U.S. Pat. Nos. 5,393,676 to Anjum et al. and 5,225,357 to Ho are cited at representative examples of this approach.
One difficulty with this approach is that fluorine (F) atoms in a polysilicon gate structure enhance boron (B) atom migration through the gate oxide to the substrate below, which tends to cause unwanted threshold voltage shifts in the corresponding FET device. This fluorine-induced migration problem typically results when atoms of the BF.sub.2 implanted in the gate polysilicon begin to diffuse due to subsequent thermal processing. To avoid this difficulty, the gate structure may be selectively masked to prevent penetration by the BF.sub.2.sup.+ ion implant species; however, this additional masking operation is generally undesirable and may prove difficult to reliably perform for gate structures having features scaled in the deep submicron regime. Thus, there is a need for a method to implant substrate source and drain regions with BF.sub.2.sup.+ ions that prevents fluorine-induced migration of boron atoms through the gate oxide.
The present invention meets this need and provides other significant advantages and benefits.